The present invention relates generally to signal processing, and more particularly to signal predistortion techniques for use in conjunction with power amplification or other nonlinear processing operations in a wireless communication system or other type of system.
As is well known, signal predistortion techniques are used in conjunction with power amplification in order to correct for undesirable effects, such as output signal distortion, spectral regrowth and adjacent channel power (ACP), that are typically associated with amplifier nonlinearity at high output power levels. In general, predistortion techniques involve distorting an input signal prior to amplification in a manner that takes into account the transfer function characteristics of the amplifier, such that the nonlinearity-related effects are at least partially canceled out in the resulting output signal.
Recently-developed signal predistortion techniques which overcome one or more problems associated with conventional techniques are described in U.S. patent application Ser. No. 09/915,042, filed Jul. 25, 2001 and entitled xe2x80x9cSystem and Method for Predistorting a Signal Using Current and Past Signal Samples,xe2x80x9d and U.S. patent application Ser. No. 09/928,628, filed Aug. 13, 2001 and entitled xe2x80x9cMultiple Stage and/or Nested Predistortion System and Method,xe2x80x9d both of which are hereby incorporated by reference herein.
Despite the considerable advances provided by the predistortion techniques described in the above-cited U.S. patent application Ser. Nos. 09/915,042 and 09/928,628, a need remains for further improvements in predistortion techniques, so as to provide additional performance enhancements in wireless communication systems and other systems employing power amplification.
In accordance with one aspect of the invention, a signal is predistorted by producing a first set of sample values using a first predistortion technique, producing a second set of sample values using a second predistortion technique different than the first predistortion technique, and combining the first and second sets of sample values to produce a predistorted signal. At least one of the first and second predistortion techniques produce the corresponding set of sample values based at least in part on a plurality of past time spaced input samples relative to a current time spaced input sample. By way of example, the first predistortion technique may be configured to produce sample output values each of which is dependent on one of a plurality of time spaced input samples and independent of any other time spaced input sample, and the second predistortion technique may be configured to produce sample output values each of which is dependent on (i) at least one of a plurality of past time spaced input samples and (ii) a current time spaced input sample, and independent of any other time spaced input sample.
In one illustrative embodiment of the invention, the predistorted signal is of the form given by the following equation having first and second addends:             y      ⁡              (        n        )              =                            ∑                      k            =            0                                K            -            1                          ⁢                              x            ⁡                          (                              n                -                k                            )                                ·                                    f              k                        ⁡                          (                              "LeftBracketingBar"                                  x                  ⁡                                      (                                          n                      -                      k                                        )                                                  "RightBracketingBar"                            )                                          +                        x          ⁡                      (            n            )                          ·                              f            xe2x80x2                    ⁡                      (                                          ∑                                  l                  =                  0                                L                            ⁢                                                c                  l                                ⁢                                  "LeftBracketingBar"                                      x                    ⁡                                          (                                              n                        -                        l                                            )                                                        "RightBracketingBar"                                                      )                                ,
where y(n) denotes the predistorted signal, x(n) denotes a corresponding input signal, fk and fxe2x80x2 denote functions, the first addend of the equation implements the first predistortion technique, the second addend of the equation implements the second predistortion technique, and cl are coefficients of one or more memory filters associated with the second predistortion technique.
In another illustrative embodiment of the invention, the predistorted signal is of the form given by the following equation having first and second addends:             y      ⁡              (        n        )              =                            ∑                      k            =            0                                K            -            1                          ⁢                              x            ⁡                          (                              n                -                k                            )                                ·                                    f              k                        ⁡                          (                              "LeftBracketingBar"                                  x                  ⁡                                      (                                          n                      -                      k                                        )                                                  "RightBracketingBar"                            )                                          +                        x          ⁡                      (            n            )                          ·                              ∑                          l              =              1                        L                    ⁢                                    f              l              xe2x80x2                        ⁡                          (                              "LeftBracketingBar"                                  x                  ⁡                                      (                                          n                      -                      l                                        )                                                  "RightBracketingBar"                            )                                            ,
where again y(n) denotes the predistorted signal, x(n) denotes a corresponding input signal, fk and flxe2x80x2 denote functions, the first addend of the equation implements the first predistortion technique, and the second addend of the equation implements the second predistortion technique.
In accordance with another aspect of the invention, the quantity l in the above equations may be permitted to have a value which is less than zero, such that one or more future time spaced input samples are utilized in the second predistortion technique.
In accordance with a further aspect of the invention, predistortion circuitry for implementing the first and second predistortion techniques may be implemented using a plurality of predistortion core circuits, each of the predistortion core circuits receiving a data input and an index input associated with a particular input sample and generating a corresponding data output. More particularly, a given one of the predistortion core circuits includes a first processing element comprising at least one of a coefficient lookup table and a polynomial generator, the first processing element receiving the index input and generating an output which is applied to a second processing element comprising a multiplier which multiplies the output of the first processing element and the data input to generate the corresponding data output. The data outputs of the predistortion core circuits correspond to at least one of the first and second sets of sample values.
The predistortion circuitry also preferably includes at least one memory finite impulse response (FIR) filter which processes at least one input sample in conjunction with at least one of the first and second predistortion techniques. For example, the memory FIR filter may generate an output which is supplied to a predistortion core circuit in the predistortion circuitry, with the predistortion core circuit producing at least a subset of one or more of the sample values.
These and other features and advantages of the present invention will become more apparent from the accompanying drawings and the following detailed description.